Chemistry Reference
In-Depth Information
wollastonite Casio
3
hedenbergite
CaFeSi
2
O
4
diopside
CaMgSi
2
O
4
ferroan salites
salites
augite
ferroan augite
ferroan
subcalcic augite
subcalcic augite
unknown
in the nature
unknown
in the nature
magnesian
pigeonite
ferroan
pigeonite
pigeonite
bronzite
enstatite
MgSiO
3
ferrosilite
FeSiO
3
hyperstène
Figure 3.22
Calcic and ferro-magnesian pyroxenes.
Ferro-magnesian pyroxenes
Orthopyroxenes form the series: MgSiO
3
enstatite-hypersthene-ferrosilite
FeSiO
3
(this last end member does not exist in nature). Protoenstatite is
a polymorph of high temperature (above 1000°C) that is stable at rela-
tively low pressure (less than 10kb). There are some very rare monoclinic
polymorphs: clinoenstatite (volcanic rocks and meteorites) and clinoferro-
silite (volcanic rocks). Bronzite is intermediate between enstatite and hyper-
stene but with a negative optic sign (hyperstene is optically positive); the
name bronzite is nowadays considered a synonym of hypersten.
Pigeonite is a ferromagnesian calcic pyroxene, poor in calcium (5-15%
of CaSiO
3
constituent). There is a miscibility gap with augite. Pigeonite is
monoclinic. It is distinguished under the microscope from other clinopy-
roxenes by the angle of the optic axes 2V which is small (0-25-30°) (2V
ranges from 25-62° in augite). Pigeonite is commonly transformed in igne-
ous rocks by a fall of temperature into hypersthene with exsolution lamellae
of augite (“inverted pigeonite”).
Calcic pyroxenes
Diopside
CaMgSi
2
O
6
-salites-
hedenbergite
CaFeSi
2
O
6
-johannsenite CaMn-
Si
2
O
6
.series.
Augites
(Ca, Mg, Fe
2+
, Al)Si
2
O
6
are very common minerals in basic
igneous rocks (gabbros, dolerites, basalts) and in ultramafic rocks; they are
somewhat less common in rocks of intermediate composition.
